refactor(ir): move declaration maps into ProgramIndex (A1.1b)

Architecture phase A1.1b — mechanical storage relocation. Move the 9
declaration-fact maps out of the Lowering state bag into ProgramIndex:

  high-fanout:   fn_ast_map, foreign_class_map, global_names, type_alias_map
  medium-fanout: struct_template_map, protocol_decl_map, protocol_ast_map,
                 module_const_map, ufcs_alias_map

168 self.<map> sites in lower.zig repointed to self.program_index.<map>;
external readers repointed too (core.zig foreign_class_map iteration;
lower.test.zig fn_ast_map / foreign_class_map). No duplicate storage, no
fallback path; zig build enforces no missed reference.

The four maps whose value types were Lowering-private pull those types into
program_index.zig as pub (GlobalInfo, StructTemplate + TemplateParam,
ProtocolDeclInfo + ProtocolMethodInfo, ModuleConstInfo); lower.zig aliases
them at file scope so call sites are unchanged.

Behavior is preserved exactly:
- per-map allocator unchanged — import_flags/fn_ast_map/global_names use the
  lowering allocator (ProgramIndex.init), the other 7 keep their page_allocator
  inline defaults;
- ProgramIndex.deinit frees only the 10 owned maps, never the borrowed
  module_scopes / import_graph;
- TypeTable.aliases still borrows &self.program_index.type_alias_map, loaned at
  lowerRoot with the same late-binding lifetime.

Extends program_index.test.zig with declaration-map round-trips (fn AST, type
alias, global, module const, foreign class, protocol decl/AST, struct template,
ufcs alias).

Registration logic (registerStructDecl / registerProtocolDecl /
registerForeignClassDecl, ...) stays in Lowering, writing through the index.

Gate green: zig build, zig build test, bash tests/run_examples.sh
(350 passed, 0 failed). lower.zig 19433 -> 19393 lines.

src/core.zig

@@ -328,7 +328,7 @@ pub const Compilation = struct {
         return module;
     }
 
-    /// Walk `lowering.foreign_class_map` and render Java sources for every
+    /// Walk `lowering.program_index.foreign_class_map` and render Java sources for every
     /// `#jni_main #jni_class("...")` declaration. Renders happen here so the
     /// AST + class-registry snapshot stay confined to the lowering pass; the
     /// downstream APK pipeline only needs `{foreign_path, java_source}` pairs.
@@ -342,7 +342,7 @@ pub const Compilation = struct {
         // and `#extends Alias` resolution.
         var registry = std.StringHashMap([]const u8).init(self.allocator);
         defer registry.deinit();
-        var it_reg = lowering.foreign_class_map.iterator();
+        var it_reg = lowering.program_index.foreign_class_map.iterator();
         while (it_reg.next()) |entry| {
             try registry.put(entry.key_ptr.*, entry.value_ptr.*.foreign_path);
         }
@@ -353,7 +353,7 @@ pub const Compilation = struct {
         // .so loading via another class.
         const lib_name = libNameFromOutputPath(self.target_config.output_path);
 
-        var it = lowering.foreign_class_map.iterator();
+        var it = lowering.program_index.foreign_class_map.iterator();
         while (it.next()) |entry| {
             const fcd = entry.value_ptr.*;
             if (!fcd.is_main) continue;